While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322–334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the organic EL element encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610–3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material-dopant, which results in an efficiency improvement and allows color tuning.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes.
A useful class of dopants is that derived from 5,6,11,12-tetraphenylnaphthacene, also referred to as rubrene. The solution spectra of these materials are typically characterized by wavelength of maximum emission, also referred to as emission λmax, in a range of 550–560 nm and are useful in organic EL devices in combination with dopants in other layers to produce white light. However, the range of light emitted by rubrene derived dopants limits the purity of the white light produced in such OLEDs. In order to achieve OLEDs that can produce higher purity white light, one needs to have the ability to increase the range of maximum emission, or emission λmax of the rubrene derived dopants so that they can be matched spectrally with the emission of dopants in the other light producing layers. Useful dopants are those that emit light in solution in the 563–650 nm range and particularly in the 565–600 nm range, also have good efficiency and sublime readily.
Yellow light is generally defined as having a wavelength range in the visible region of the electromagnetic spectrum of 570–590 nm, orange light 590–630 nm and red light 630–700 nm, as defined by Dr. R. W. G. Hunt in The Reproduction of Colour in Photography, Printing & Television, 4th Edition 1987, Fountain Press, page 4. When light has a spectral profile that overlaps these ranges, to whatever degree, it is loosely referred to as yellow-orange or orange-red.
U.S. Pat. No. 6,387,547; U.S. Pat. No. 6,399,223; EP 1,148,109A2; JP20001156290A; and JP 04335087 teaches the use of rubrene derivatives containing either 2 phenyl groups on one end ring of the rubrene structure or 4 phenyl groups on both end rings. There is no teaching of single branched alkyl or non-aromatic carbocyclic groups on each end ring of the rubrene structure.
Publication “A New Yellow Fluorescent Dopant for High-Efficiency OLEDs”, 11th International Workshop on Inorganic and Organic Electroluminescence & 2002 International Conference on the Science and Technology of Emissive Displays and Lighting, September 2002, Session 4, El2002 Ghent, Ghent University, Ghent, Belgium, discloses rubrene derivative TBRb. The disclosed TBRb compound is a yellow fluorescent dopant for OLED devices.
However, these rubrene derivatives do not produce the desired emission λmax. Thus devices containing these rubrene derivatives fail to provide white OLED devices with high color purity.
The problem to be solved is to provide a dopant compound for a light-emitting layer of an OLED device that provides white light purity, and high luminance efficiency.